In the art of magnetic resonance imaging (MRI) it is desirable to have a radio frequency (RF) magnetic resonance (MR) excitation field B1 of a high degree of homogeneity for exciting nuclei of or within the subject of interest. Multi-channel transmit MRI has been accepted as a standard method of operating volume RF coils to achieve a relatively uniform RF magnetic excitation field B1. Compared with a single-channel mode of operation, a two-channel transmit technique results in a significantly increased B1 homogeneity.
In known multi-channel transmit systems, an RF power signal is typically supplied to an RF coil arrangement via RF drive ports being connected to individual resonator elements of the RF coil arrangement. Two-channel transmit MR systems typically use two independent RF transmit chains and amplifiers for applying the RF power signals to the RF drive ports of the coil arrangement. The RF power applied to the different RF drive ports can be controlled individually in order to optimize the homogeneity of the RF field (so-called ‘RF shimming’).
A conventional RF transmit system is schematically illustrated in FIG. 2. Herein, the RF transmit system comprises an RF coil arrangement CA that is designed as a birdcage coil having a pair of circular loop members L1, L2 and a plurality of 16 interconnecting rungs R, as is well known in the art. The RF coil arrangement further includes a pair of drive ports DP that are electrically connected to rungs or to the circular loop members and are arranged at locations that are spaced by an angular distance of 90° in an azimuthal direction about a birdcage coil center axis A. RF power from two RF power sources P1, P2 is provided to the pair of drive ports DP to excite two orthogonal modes of the birdcage coil.
By way of example, international application WO 2012/093321 A1 describes a multi-channel (e.g. quadrature) MRI transmit system in which RF power amplifiers having different power capabilities are used in different transmit channels. This results in reduced system costs, due to the avoidance of an unused excess of RF power capability when the power demand for obtaining a homogeneous B1 field (RF shimming) is asymmetric and the asymmetry is qualitatively the same for different imaging applications. The multi-channel transmit unit may also comprise a commutator which enables to selectively connect each RF power amplifier to each drive port of transmit coil arrangement (e.g. a birdcage coil).